This invention relates to a PID controlling meter, and more particularly, to a technique for controlling overshoot generated when a power source is closed.
As is well known, PID controlling meters have advantages in that less steady state deviation occurs and in that correction of disturbance can be quickly accomplished, and therefore, PID controlling meters are extensively used for various process controls such as temperature control.
In PID controlling meters of this kind, over-shoot often occurs in control operations performed when a power source is closed. This results from the fact that when the power source is cut off, the integrated values so far presented are lost and when the power source is again closed, integrating operation restarts from an initial integrating value (normally, zero) and as a consequence, the number of required integration operations increases. To minimize such over-shoot and to shift the condition to the previous stabilized control condition quickly, the integrated value from outside need merely be corrected, but to perform a correcting operation upon every closure of the power source is very cumbersome and operability is poor.
In view of the foregoing, in prior art PID controlling meters, an antireset wind up function is utilized wherein an amount of upper limit operation and an amount of lower limit operation in the range capable of being integrated are determined from the amount of operation under the stabilized control condition, and the integrating operation is effected only when the amount of operation is within said range when the power source is reclosed, wherby the over-shoot resulting from over-integration is minimized to eliminate the necessity of a correcting operation by the user.
However, PID controlling meters which use anti-reset wind-up functions have the following problems.
First, since the integrating operation is not performed when the amount of operation is other than the above described set range, said set range has to be extended to prevent an increase in steady-state deviation due to the influence of disturbances. This is not favorable in terms of the initial object which is to minimize overshoot. In addition, in order to determine the above described set range, a high degree of expertise is required using data for obtaining the stabilized control condition in respect of objects to be controlled and also data concerning the condition disturbance and bringing appropriate judgment on the basis of said data. Thus, it is not possible for the user to simply make a decision. Finally, two kinds of settings are necessary, thus making the structure of the controlling meter quite complex.